1. Field of the Invention:
The present invention relates to novel macrocrystals of .alpha.-alumina, or aluminum oxide or corundum, in the form of hexagonal platelets and to a process for the preparation of such novel crystals from aluminum oxide or aluminum hydroxide.
2. Description of the Prior Art:
The preparation of macrocrystals of alumina from aluminum hydroxide is known to this art. Thus, French Patent No. 2,441,584 describes macrocrystals of .alpha.-alumina in the form of hexagonal platelets having a particle size ranging from 16 to 250 .mu.m and a diameter/thickness ratio ranging from 3/1 to 7/1, such macrocrystals being prepared by calcination of aluminum hydroxide at a temperature ranging from 1,200.degree. C. to 1,450.degree. C. in the presence of a mineralizing agent containing 0.001% to 0.5% by weight of fluorinated compounds.
Published German Patent Application DE-OS 2,623,482 describes the preparation of primary crystals having an average size ranging from 16 to 25 .mu.m by calcination of aluminum hydroxide in the presence of at least one fluorinated salt and a vanadium salt.
According to the above '584 French patent (page 2, lines 19-22), increasing the temperature or the use of other mineralizing agents has no or virtually no influence on the size and the shape of the crystals.
Moreover, Keiji Daimon and Etsun Kato, in Yogyo Kvokai Shi, 94, [3], pp. 380-382 [78-80] (1986), describes the production of hexagonal platelets of .alpha.-alumina having a diameter ranging from 1.5 to 40 .mu.m from hydrated aluminum sulfate in the presence of aluminum trifluoride.
Published Japanese Patent Application No. 60/54,916 describes the preparation of platelets having an average diameter of 1.7 .mu.m and a thickness of 0.18 .mu.m from a mixture of hydrated aluminum sulfate and an alkali metal carbonate.
Published Japanese Patent Application No. 60/38,486 describes the preparation of platelets having a thickness of less than 5 .mu.m and a diameter (derived from the diameter/thickness ratio) ranging from 25 to 150 .mu.m.
And Russian Patent No. 416,313 describes the preparation of hexagonal platelets of alumina which have a diameter ranging from 4 to 12 .mu.m by thermal dissociation of an aluminum compound in the presence of hexagonal alumina seed material at 1,170.degree. C.
Finally, B. P. Locsel, in INTERCERAM, No. 3 (1981), describes the preparation of .alpha.-alumina crystals having a diameter/thickness ratio ranging from 6 to 10 from bauxite having a low ferric oxide content, in the presence of aluminum trifluoride.